Regulating system



Jan- 5, 1943. J. F. KovALsKY REGULTING SYSTEM Filed Dec. 2, 1941 2 Sheets-Sheet l .2 m 9 ,X 5 4 f o suxmk vuvb ...il .TTT-.7.1.7--

ORNEY Jan. 5, 1943. J. F. KovALsKY REGULATING SYSTEM Filed Dec. 2, 1941 2 Sheets-Sheet 2 H//ou Chanyc In INVENTOR fafepflf/ava/.s-Ay

WITNESSES:

Patented Jan. 5, 1943 REGULATING SYSTEM Joseph F. Kovalsky, Turtle Creek, Pa., assignor to Westinghouse Electric a: Manufacturing Company, East Pittsburgh, Pa., a corporation oi' Pennsylvania Application December 2, 1941, Serial No. 421,309

6 Claims.

This invention relates to electrical apparatus and, particularly, to voltage regulating systems for dynamo-electric machines.

Voltage regulators are employed with dynamoelectric machines for maintaining constant voltage across a given load and have been so developed/that they are capable of maintaining such constant voltage for a given range of operating temperatures ranging from about '72 F. upwardly. However, in some cases, the voltage regulator must operate over a range of temperatures from F. no load to approximately 120 F. full load.

The regulators and regulating systems provided heretofore are not satisfactory for operation over such a wide range of operating temperatures as the resistance characteristics oi the regulators and regulating systems change with changing temperatures. Under these conditions, the resistance of the voltage regulating apparatus is not constant enough to give stable operation oi the voltage regulator to maintain a constant voltage on the dynamo-electric machine, errors as large as 8 to 10% in the controlled voltage being obtained in operating the prior art regulators and regulating systems over operating temperatures ranging from 0 to 120 F. ambient temperature.

An object of this invention is to provide in a regulating system for a dynamo-electric machine for compensating for changes in resistance of the regulator occasioned by changes in operating temperature, and thereby maintain substantially constant voltage on the machine.

Another object of this invention is to provide in a temperature responsive device for a regulating system for connecting dierent sections of a plurality of resistor units in parallel circuit relation as changes in temperature occur to give the device predetermined operating characteristics.

Other objects of this invention will become apparent from the following description when taken in conjunction with the accompanying drawings, in which:

Figure 1 is a view in elevation of a temperature responsive device embodying the teachings of this invention;

Fig. 2 is a side elevational view partly in section of the apparatus of Fig. 1;

Fig. 3 is a side elevationalview partly in section of another embodiment of the temperature responsive device;

Fig. 4 is a graph, the curves of which illustrate errors of the type which occur in regulating ap- (Cl. ITI-119) paratus occasioned by change in'operating temperatures;

Fig. 5 is a graph, the curve of which illustrates a change in resistance with a change inl temperature over a period of time which is found in certain apparatus generally utilized in regulating systems for transposing alternating current to direct current;

Fig. 6 is a graph, the curve of which illustrates the change of resistance with a change in temperature for certain types of regulators;

Fig. 7 is a schematic representation of one of the embodiments of the temperature responsive device utilized in accordance with this invention for compensating for errors of the type illustrated in Fig. 4;

Fig. 8 is a graph, the curves of which illustrate the resultant resistance characteristics oi! the temperature responsive-device of Fig. l obtained by changing the connections of the resistors.

Fig. 9 is a schematic representation of another embodiment oi the temperature responsive device utilized in accordance with this invention for compensating for errors of the type illustrated in Fig. 6;

Fig. 10 is a graph, the curves of which illustrate the resultant resistance characteristics of the temperature responsive device of Fig. 9 ob- 'tained by changing the connections of the resistors; and

Fig. 11 is a schematic diagram of a voltage regulating system embodying the teachings of this invention.

Referring to Figs. 1 and 2, this invention is illustrated by reference to a temperature responsive device I0 utilized for compensating for resistance changes in regulating systems in accordance with this invention. The temperature responsive device III comprises a. plurality of ad- 40 instable resistors I2 and Il and a temperature responsive element I6 secured to a suitable base member I 8.

Each of the adjustable resistors I2 and I4 are of standard design, the resistorl I2 being of the sliding contact type and provided with end terminals 2l and 22 and a mid terminal 2l. The

mid terminal 24 is in sliding electrical engagement with a conducting sleeve 21 insulated from but carried by a spindle 39. A movable contact finger 28 is in electrical engagement with the conducting sleeve 31 disposed to be actuated as the spindle is moved to slidably engage the resistor I2. In order to mount the resistor I2 on the base I8, the resistor I2 is carried in a suitable B5 housing 2l as illustrated in Fig. 2 which is mounted on a hollow stud 80 which projects through a suitable opening in the base I8. The nuts 32 carried on the hollow stud 30 on each side of the base member I8 are tightened to secure the resistor in operative position with respect to the base member.

'I'he adjustable resistor I4 may be of any standard make, and in this embodiment comprises a spiral wound resistor element 34 positioned in a protective insulating housing 36 and provided with end terminals 38 and 46 and a slidable mid terminal 42 disposed to engage the turns of the resistor element I4. In practice, the resistor I4 is mounted on the base member I8 by means of the bolt 44 extending through the resistor I4 and through a suitable opening in the base I8, a nut 46 being provided for engaging the end of the bolt 44 and holding the resistor I4 in its operative position.

The thermal responsive element I6 is in the form of a spiral bimetal having one'of its ends secured to a bracket 48 as by means of the screw 50. 'Ihe other end of the bracket is secured to the base member I8 in any suitable manner or as shown, by the screw 52. In order to control or regulate the movement of the contact nger 26 of the resistor I2, the enti of the spindle 89 opposite the end which carries the sleeve 31 and the contact finger 26 is disposed to extend through the hollow stud 36 and is provided with a slot 54, as illustrated in Fig. 2. The inner end of the spiral bimetal element I6 is then positioned inthe slot 54 of the spindle and secured thereto in any suitable manner. In practice, a housing 56 of insulating material is provided about the bimetal ele'ment I6 and is secured to the base member I 8, as by means of the screw members 58 projectingtherethrough.

In an embodiment of this invention where it is desired to apply heat in a predetermined manner to the bimetal element I6, a heater element 60 is provided within the housing. As illustrated in Fig. 3, where the heater element 66 is utilized, the housing is modified somewhat, the cover being adapted to carry the heater element 66. In this embodiment, the heater element 60 is mounted on conducting studs 62 which project through the cover of the housing 56 and are in electrical contact with terminals 64 provided externally of the cover.

As will be explained more fully hereinafter, it is necessary to provide a time delay in the heating of the bimetallic element I6 to the predetermined temperature of the heat developed by the heater element 68. In order to provide such a time delay or thermal lag, a plate 66 of` thermally conducting material, such as copper, is disposed within the housing 56 between the heater element 60 and the bimetallic member I6, suitable openings (not shown) being provided in the plate 66 for pennitting the bolts 58 to pass therethrough. The thermal lag obtained through the use of the copper plate 66 will depend upon the thickness of the copper plate, the thickness readily being determined experimentally, depending upon the heat developed by the heating element 66 and the thermal lag which it is desired to obtain before the bimetal element I6 is heated to the predetermined temperature of the heating means.

Referring to Fig. 1l of the drawing, the thermal responsive device I6 is utilized in a voltage regulating system for'maintaining the voltage of an alternating current generator 66 substantially constant. As illustrated, the generator 66 comt8 prises the armature winding 'I0 and the separately excited eld winding l2' and is disposed to be driven by any suitable source of power, such as a motor (not shown). The generator 68 is connected by conductors 'I4 and I6 to any suitable load (not shown) for delivering power thereto at a constant voltage.

The eld windings 'I2 are connected to a suitable source of power, such as the battery 'I8, through a variable rheostat 86, the rheostat being disposedto be operated to control the energization of the field winding.

As illustrated, the rheostat 86 comprises a plurality of resistor sections disposed to be connected in series circuit relation with the field winding 12, the operation of the rheostat 80 being controlled by a switching apparatus 82 formed of a plurality of spring biased leaf members 84 of the type fully described in Patent No. 2,246,- 301, issued June 17, 1941, to C. R. Hanna et al. and assigned to the same assignee as this invention. l

In order to eiect a switching operation of the.

switching apparatus 82-an electromagnetic de-" vice is provided. The electromagnetic device may be of any suitable construction and comprises a core member 86 having an actuating winding 88 thereon and an armature member 96 which carries a driver member 92 disposed for movement when the actuating winding 88 is energized.

In order that the operation of the switching apparatus 82 may be a function of the voltage of the generator Ill, the actuating winding 88 of the electromagnetic device is connected across the generator 68 through a copper-oxide rectier 94 which is utilized for delivering direct current to the winding as. As illustrated, the thermal responsive device I0 is connected in the actuating winding circuit between the copperoxide rectier 94 and the actuating winding 88. The heating element 6U of the thermal responsive device I0 is also connected across the conductors 14 and 'I6 to be energized at the same time that the actuating winding circuit is energized.

In order to understand the operation of the subject matter of this invention, reference may now be had to the curves of Fig. 4 of the drawings. In this iigure, curve I represents the temperature error or change of resistance of the copper-oxide rectifier 94 generally employed in regulating systems over a range of ambient temperatures after the rectier is heated to its ultimate operating temperature while curve 3 represents the temperature error, under the same conditions, of the copper windings of the actuating winding 88 and other apparatus such as a feed-back transformer (not shown) generally included in the actuating winding circuit f the regulator. As is evident from Fig. 4, the temperature errors represented by curves I and 3 are in opposition, and if the apparatus is properly designed, a resultant change in resistance for the actuating Winding 68 and the circuit including the copperoxide rectifier 9i can be had, as represented by the curve 5 of Fig. 4. As illustrated, the resultant resistance of such a circuit is a non-linear'variable for vchanges in temperature ranging from 0 F. up to about 75 F. From 75 F. up to about 1405. F., the resultant resistance of the actuating winding circuit including the rectier is substantially a constant, so that substantially no temperature error is found for this portion of the range of operating temperatures. Because 'of the change in resistance, as represented by curve 5, between the operating temperatures o! up to about 75 F., it is necessary to compensate for such changes as by introducing another change of resistance in the actuating winding 30, as represented by the curve 1, which shall be equal and opposite to the change, as represented by curve 5, for changes in operating temperature up to '15 F.

Referring to Fig. 5, there is illustrated another type of error which exists where apparatus such as the copper-oxide rectiner 34 is employed in the actuating winding circuit of the regulator when such apparatus is utilized in cold climates. As illustrated by curve 3, the resistance of the copper-oxide rectifier 3i decreases for an increase in temperature at a constant ambient temperature up to the ultimate operating temperature of the apparatus, such decrease being found to exist for periods of time of up to approximately two hours before the rectifier becomes so heated due to the flow of current to the actuating winding 38 as to eliminate this type of error. It is, therefore, necessary to introduce a compensating resistance which has a change in resistance equal and opposite to the change in resistance of the copper-oxide rectiiler for this type of error. Such compensating resistance can be represented by the curve II, it being noted that when the changes in resistance represented by curves 3 and II are balanced that the combined resultant resistance will be substantially a constant for the period of time in which it takes the rectifier to heat up to its ultimate temperature.

As distinguished from the error represented by curve 5 of Fig. 4, it is possible that the resultant resistance changes due to change in ambient temperature after the apparatus is heated to its ultimate operating temperature might have a temperature error, as represented by the curve I3 of Fig. 6. Such temperature errors are found where the actuating winding circuit of the regulator includes apparatus in which the ratio of the copper windings to the resistance of the copper-oxide rectifier is such that the output at high temperature has a rising characteristic. In such cases, it is, therefore, necessary to introduce a compensating resistance in the actuating winding circuit which will have a change in resistance, as represented by the curve I5 of Fig. 6 and have a drooping eiiect at the high temperatures.

Referring to Fig. `'7, there is schematically represented a temperature responsive device I0 suitable for compensating for the change in resistance of the actuating winding circuit represented by the curve 5 of Fig. 4. In this figure, the resistor elements I2 and 34 are connected in parallel circuit relation, a predetermined section of the resistor element 34 being connected in the circuit by adjusting the terminal or tap 42. The

amount of the resistor element 34 connected in the circuit is determined by the slope of the curv 5 of Fig. 4, since the non-linear curve 1 representing the resultant resistance of the parallel connected resistors is to be equal and opposite the curve 5. Having the slope of curves 5 and 1 the amount of resistor element 34 to be connected in the circuit can readily be determined from the equation R1 -i-Rz where Ri represents the portion of the resistor 34 in the parallel circuit, and Rz represents the portion of the resistor i2 in the parallel circuit.

In the embodiment illustrated in Fig. '1, if the apparatus is operating at a temperature of about 0 F., the bimetallic element is in its cold position where the contacting ringer 26 is adjacent the terminal 2li so that very little of the resistor i2 is connected in the parallel circuit'of the temperature responsive device. The adjustable terminal 42 of the resistor element 34 having been adjusted to some predetermined position to ccnnect a predetermined amount of the resistor 34 in the parallel circuit depending upon the slope of the curve 5, any increase in the ambient temperature which would change the resistance of the actuating winding circuit including the rectifier also aifects the bimetal member to cause it to expand and effect a counterclockwise .movement of the contact ringer 26 along the resistor I2 to connect additional amounts or sections of the resistor I2 in parallel with the portion of the resistor 34 to give a predetermined non-linear resistance change which follows the slope of curve 1 of Fig. 4.

The predetermined non-linear resistance which is obtained through the use of the temperature responsive device I 0 of Fig. 'Z for given changes in the amount of the resistance I2 connected in parallel in the circuit for definite and different amounts of the resistor 34 is represented by the curves of Fig. 8. In this graph, each of curves I1, I3, 2l, 23, and 25 represents the resultant resistance obtained by a change in the connections to resistor I2, as the contact linger 26 moves from terminal 20 to terminal 22 for diierent amounts of the resistor 34 connected in the parallel circuit. As illustrated, as the terminal 42 is moved along the resistor 34 to connect larger amounts of the resistor 34 in the parallel circuit with resistor I2, the slope of the curves I1 through 25 becomes steeper, until with the amount of resistor 34 being an infinity, the curve 25 becomes a straight line.

If the regulating winding circuit, however, has such a ratio of copper to the resistance of the rectiner that theoutput at high temperature has a rising characteristic, as illustrated by curve I3 of Fig. 6, then the temperature responsive device Il illustrated in Fig. 9 is employed for compensating for the change in resistance of the actuating winding circuit. In this embodiment, the resistor element I2 is so connected in parallel circuit relation with the resistor element 34, that, in effect. as the contact finger 28 is actuated from terminal 2l to terminal 22, the resistor element I2 forms two variable resistor sections which are connected in parallel circuit relation with one another and with the resistor element 34. In this embodiment, with the regulating system operating at temperatures from 0 F. up to 120 F., as the ambient temperature increases from 0 F., the bimetal element I6 actuates the contact finger 26 along the resistor element I2 towards the terminal 22. The operation of the contact finger 26 effects a simultaneous and inverse change in the amount of resistance of the two resistor sections formed by the resistor I2 which are connected in parallel with the resistor 34.

l The slope of the curve obtained by such a change can be predicted by the equation R -1 llRi'i'l/Rz'tl/Rs the resultant resistance represented by the curves 23, 3i, 33, and 35 of Fig. 10 being obtained for different values of the resistor 34 connected in the circuit for changes in the ambient temperature. In l'this equation, Ri represents the portion of the resistor 34 connected in the parallel circuit, Ra represents the resistor section between terminal 20 andcontact linger 26 of the resistor element I2, and R3 represents the resistor section between the terminal 22 and contact nger 26 of the resistor l2 which are connected in the parallel circuit. As illustrated for dilerent values of the resistor 34, a resultant resistance is obtained having diiierent slopes and diierent peaks of resistance, so that a resultant resistance having a predetermined change in resistance characteristic which will match the curve i5 of Fig. 6 can be obtained.

Again referring to Fig. 11, assuming that the ratio of the copper windings in the lactuating winding circuit to the resistance of the copperoxide rectier 941 included in the circuit is such that the output at high temperature has a drooping characteristic, as represented by the curve 5 of Fig. 4, the thermal responsive device lli of Fig. 7 is connected in the actuating winding circuit for compensating for the change in resistance of the actuating winding circuit including the rectier.

In operation, assuming that the generator Bt is driven at a constant speed by some suitable source of power, such as a motor not shown, the voltage across the generator for a given load is a constant. In starting the generator 60, the regulating system nrst becomes active as the generator S8 delivers power to a load, not shown. Thus, the actuating winding 6% is energized in accordance with the voltage across the generator B, and since the regulating system may be energized at as low a temperature as below 0 F., the characteristics of the copper-oxide rectifier 94 included in the actuating winding circuit of the regulating system change as the actuating winding 8S becomes energized. However, because of the construction of the rectifier, such changes in its resistance characteristics is slow, generally taking a period of about two hours before the rectier 8d becomes heated to its ultimate temperature of 0 F. orv

higher.

Therefore, in the embodiment illustrated in Fig. 1l, the heater element 60 of the temperature responsive device i0 is connected across the conductors 'it and iii, so as to be energized at the same time that the actuating winding circuit of f' the regulating system is energized. Because of the inclusion of the copper plate 6@ between the bimetal element i6 and the heater element G6, a thermal lag equivalent .to the period of time in which it takes the copper-oxide rectier 9G to become heated te its ultimate temperature of 0 F. or higher is introduced in the heating of the bimetal element it to the ultimate temperature of the heating element Gt. As the bimetal element it is heated at approximately the same rate in which the rectifier 9d becomes heated, the contacting ringer 2t of the resistor element i2 is actuated to connect additional portions of the resistor l2 in the parallel circuit with resistor element 30 to give a resultant change in resistance of the parallel connected resistors l2 and 3d which is equal and opposite to the change in the resistance of the rectifier @d during the period of time in which the rectier is heated to its ultimate temperature. Thus, in eiect, the temperature responsive device l0 compensates for the change in resistance of the copper-oxide rectifier, as the rectiner becomes heated to its ultimate temperature of 0 F. or higher.

Assuming that the generator is delivering power to a. load, now shown, and that the copperoxide rectier 94 and the windings of the actuating winding circuit have become heated to their ultimate temperature, if for any reason the load should change so as to eii'ect a. decrease in the voltage across the generator, then the actuating winding 88 of the electromagnetic device 86 is so deenergized that the driver member 92 is moved to actuate the spring bias switching members 84 of the switching apparatus 82 to `shunt a predetermined number of the sections of the eld winding rheostat 80 to increase the energization of the field winding 'I2 and thereby effect an increase in the voltage across the generator 63.

If during the operation of the regulating system after the apparatus is heated to its ultimate temperature of 0 F. or higher, the ambient temperature of the regulating system is changed, as by reason of an increase in the room temperature in which the apparatus is located, the resultant resistance'of the actuating winding circuit including the rectifier is also changed so as to normally affect the energization of the actuating winding 83. However, by reason of the temperature responsive device l@ connected in the actuating winding circuit, a change in the ambient temperature also aeots the bimetal element i6 to actuate the contact finger 2li to change the connection to the resistor it, whereby the resultant resistance of the temperature responsive device Iii also changes in an amount equal and opposite to any change in the actuating winding circuit including the rectifier ed. In eiect, such changes in the resultant resistance of the temperature responsive device being equal and opposite to the changes in the resultant resistance of the actuating winding circuit including the rectier @E gives the actuating winding circuit a substantially constant resultant resistance If the load supplied by the generator 68 is decreased so as to eect an increase in the voltage across the generator, then the actuating Winding 88 is so energized as to actuate the armature 90 and thedriver member 92 carried thereby in a direction to effect the release oi the spring biased leaf members Bil of the switching apparatus 82 to progressively connect additional sections of the eld winding rheostat 80 in circuit with the eld winding 'i2 to decrease the energization of the eld winding 'l2 and, consequently, decrease the voltage across the generator E8. The temperature responsive device l@ functions in the regulating system for all operations thereof to give a resultant resistance in the actuating winding circuit thereof which is substantially constant for all changes in temperature whether such changes are due to heating of the apparatus in the regulating winding circuit or due to changes in ambient temperature.

By utilizing the temperature responsive device inthe regulating system, as described hereinbefore, substantially constant voltage is obtained acrom the generator, the voltage of which is being regulated. In practice, where the temperature changes range from 0 F. up to between 120 and 140 F., the voltage of the generator is found to be maintained constant within plus or minus .5%. The apparatus is economical, being constructed of standard equipment and functions to give a regulating system superior to the systems known heretofore.

Although this invention has been described with reference to a, particular embodiment thereof, it is, of course, not to be limited thereto except insofar as is necessitated by the scope of the appended claims.

I claim as my invention:

1. In a voltage regulating system. in combination, a generator the voltage of which is to be regulated, the generator being provided with a eld winding, a rheostat for controlling the voltage impressed on the iield winding, means for actuating the rheostat, the rheostat actuating means comprising a winding connected in circuit across the generator, the winding and circuit across the generator having a resistance variable with changes in temperature. and means connected in the actuating winding circuit for compensating for the change in resistance of the winding and circuit to maintain the eiiective resistance of the actuating winding circuit substantially constant over a wide range of operating temperatures, the compensating means comprising a plurality of resistors connected in parallel circuit relation with one another, the connection to one of the resistors being adjustable, and means responsive to changes in temperature for effecting the adjustment oi the connection to said one of the resistors to change the resistance connected in parallel circuit relation with the other resistor.

2. In a voltage regulating system, in combination, a generator the voltage of which is to be regulated, the generator being provided with a field winding, rheostat for controlling the voltage impressed on the field winding, ,means for actuating the rheostat, the rheostat actuating means comprising a. winding connected in circuit across the generator, the winding and circuit across the generator having a resistance variable with changes in temperature and means connected in the actuating winding circuit for compensating for change in resistance of the winding and circuit to maintain the eiiective resistance of the actuating winding circuit substantially constant over a wide range of operating temperatures, the compensating means comprising a plurality of resistors connected in parallel circuit relation with one another, the connection to one ot the resistors being adjustable, means responsive to changes in temperature for eiIecting the adjustment oi! the connection to said one of the resistors to change the resistance connected in parallel circuit relation with the other resistor, a heating element associated with the temperature responsive means disposed to supply heat at a predetermined temperature thereto, and means disposed between the temperature responsive means and the heating element to effect a time delay in heating the temperature responsive means to the predetermined temperature of the heat supplied by the heating means.

3. In a voltage regulating system, in combination, a generator the voltage of which is to be regulated, the generator being provided with a iield winding, a rheostat for controlling the voltage impressed on the rleld winding, means for actuating the rheostat, the rheostat actuating means comprising a winding connected by a circuit including a rectier across the generator, the winding and the circuit including the rectiner having a resultant resistance variable over a portion of the range of operating temperatures of the actuating means, and means connected 7o in the actuating winding circuit for compensating for the change in resistance of the circuit to maintain the resistance thereof substantially constant over a wide range of operating temperatures, the compensating means comprising a plurality of resistors connected in parallel circuit relation with one another, the connection to one of the resistors being adjustable, and means responsive to changes in temperature for effecting the adjustment of the connection to said one of the resistors to change the resistance connected in parallel circuit relation with the other resistor to give a resultant resistance for the parallel connected resistors which is variable over the same portion of the range of operating temperatures as the resultant resistance of the actuating winding circuit, the change in the resultant resistance of the parallel connected resistors being substantially equal and opposite to the change in the resultant resistance of the actuating winding circuit over the same range of operating temperatures.

4. In a voltage regulating system, in combination, a generator the voltage of which is to be regulated, the generator being provided with a field winding, a rheostat for controlling the voltafge impressed on the ileld winding, means for actuating the rheostat, the rheostat actuating means comprising a winding connected by a circuit including a rectifier across the generator, the winding and circuit including the rectifier having a resultant resistance variable over a portion of the range of operating temperatures of the actuating means, and means connected in the actuating winding circuit for compensating for the change in resistance of the circuit, the compensating means comprising a plurality of resistors connected in parallel circuit relation with one another, the connection to one o! the resistors being adjustable, means responsive to changes in temperature for effecting the adjustment of the connection to said one of the resistors to change the resistance connected in parallel circuit relation with the other resistor, a heating means associated with the temperature responsive means disposed to heat it to a predetermined temperature, and means disposed between the temperature responsive means and the heating means for providing a thermal lag in the heating oi the temperature responsive means to the predetermined temperature by the heating means, the temperature responsive means being effective tor changing the resultant resistance of the parallel connected resistors in opposition to the change of the resultant resistance o! the actuating winding and circuit includingthe rectifier to maintain the effective resistance of the circuit substantially constant over a. wide range of operating temperatures.

5. In a voltage regulating system, in combination, a generator the voltage of which is to be regulated, the generator being provided with a field winding, a rheostat for controlling the voltage impressed on the iield winding, means for actuating the rheostat, the rheostat actuating means comprising a winding connected by a circuit including a rectifier across the generator. the winding and circuit including the rectiiler having a resultant resistance variable over a portion of the range oi operating temperatures of the actuating means, and means connected in the actuating winding circuit for compensating for the change in resistance o! the circuit, the compensating means comprising three resistors connected in parallel circuit relation with one another, the connection to two of the resistors being adjustable, and means responsive to changes in temperature for effecting the adjustment of the connection to said two of the re- 76 sistors to simultaneously and inversely change the resistance of said two resistors to change the resultant resistance of the three parallel connected resistors, the change in the resultant resistance of the parallel connected resistors being substantially equal and opposite to the change in the resultant resistance of the actuating winding and circuit including the rectier to maintain the eective resistance of the circuit substantially constant over a wide range of operating temperatuxes.

6. In a voltage regulating system, in combina tion, a generator the voltage of which is to be regulated. the generator being provided with a ileld winding, a rheostat for controlling the voltage impressed on the iield winding. means for actuating the rheostat, the rheostat actuating means comprising a winding connected by a circuit including a rectifier across the generator, the winding and circuit including the rectier having a resultant 'resistance variable over a portion of the range of operating'temperatures of the actuating means, and means connected in the actuating winding circuit for compensating for the change in resistance of the circuit, the ccm-a pensating means comprising three resistors come nected in parallel circuit relation with one another, the connection to two of the resistors being adjustable, means responsive to changes in temperature for eiecting the adjustment of the connection to said two of the resistors to simultaneously and inversely change the resistance of said two resistors to change the resultant resistance of the three parallel connected resistors, a heating means associated with the temperature responsive means .disposed to heat it to a predetermined temperature, and means disposed between the temperature responsive means and the heating means for providing a thermal lag in the heating of the temperature responsive means to the predetermined temperature by the heating means, the temperature responsive means being eiective for changing vthe resultant resistance of the parallel connected resistors in opposition to the change of the resultant resistance of the actuating winding and circuit including the rectier to maintain the eective resistance oi the circuit substantially constant over a wide range of operating tem= peratures.

JOSWH F. KOVALSKY. 

